Commander 112A pilot reported airspeed decayed below stall speed after encountering inflight icing during cruise. Pilot returned to departure airport and landed safely.

Narrative

Purpose of flight was an IPC (Instrument Proficiency Check). Departed ZZZ1; bound for ZZZ on IFR flight plan with clearance to 3000 MSL. Pitot heat was switched on during takeoff climb. After entering the cloud layer at about 2200 MSL; light rime icing began accumulating. Rate of accumulation was deemed insignificant. First planned approach was ILSXX at ZZZ. Near the ZZZZZ intersection; indicated airspeed began decreasing very slowly. While determining if the airspeed loss was due to performance loss; the indicated airspeed began to decrease rapidly and fell well below aircraft stall speed in the clean configuration. I instructed the Pilot Flying to pitch down and descend assuming the airspeed loss was real. Coincident with this action; a request to ATC was made to return to ZZZ1. ATC issued a clearance to ZZZZZ1 and the RNAVXY approach to ZZZ1. We descend below the MVA of 2800 MSL and ATC directed us to maintain 2800. Unsure yet whether the airspeed loss was real; we continued the descent into VFR conditions at about 2200 MSL. ATC cancelled the approach clearance and shortly after that we cancelled our IFR clearance and returned to ZZZ1 VFR without incident. GPS ground speed was used as a proxy for indicated airspeed.After shutdown; the pitot heat was checked. While some heating was detected it did not seem to be robust enough to me. The Flying Pilot will have the pitot heat operation verified by a mechanic.